1. Field of the Invention
The present invention relates to a method of semiconductor manufacturing in which N-type doping is accomplished by the implantation of ion beams formed from ionized molecules, said ions being of the form AnHx+, or AnRHx+, where n and x are integers with n greater than 4 and x greater than or equal to 0, and A is either As or P, and R is a molecule not containing phosphorus or arsenic, which is not injurious to the implantation process.
2. Description of the Prior Art
The Ion Implantation Process
The fabrication of semiconductor devices involves, in part, the introduction of specified impurities into the semiconductor substrate to form doped regions. The impurity elements are selected to bond appropriately with the semiconductor material so as to create electrical carriers. This introduction alters the electrical conductivity of the semiconductor material in the “doped” region. The concentration of dopant impurities so introduced determines the electrical conductivity of the resultant region. The electrical carriers can either be electrons (generated by N-type dopants) or holes (generated by P-type dopants). Many such N- and P-type impurity regions must be created to form transistor structures, isolation structures and other such electronic structures, which function collectively as a semiconductor device.
Ion implantation is the conventional method of introducing dopants into a semiconductor substrate. In ion implantation, a feed material containing the desired element is introduced into an ion source and energy is supplied to ionize the feed material, creating ions which contain the dopant element. For example, in silicon the elements As, P, and Sb are donors or N-type dopants, while B and In are acceptors or P-type dopants. An accelerating electric field is provided to extract and accelerate the ions, thus creating an ion beam. Typically, the ions contain a positive charge. However, in certain cases negatively-charged ions may be used. Mass analysis is used to select the exact species to be implanted. The mass-analyzed in beam may subsequently pass through ion optics which alter its final velocity or change its spatial distribution prior to being directed into a semiconductor substrate or work piece. The accelerated ions possess a well-defined kinetic energy which allows the ions to penetrate the target to a predetermined depth. Both the energy and mass of the ions determine their depth of penetration into the target. Higher energy and/or lower mass ions allow deeper penetration into the target due to their greater velocity. The ion implantation system is constructed to carefully control the critical variables in the implantation process. Critical variables include: the ion acceleration, ion mass, ion beam current (electrical charge per unit time), and ion dose at the target (total number of ions per unit area that penetrate into the target). Beam angular divergence (the variation in the angles at which the ions strike the substrate) and beam spatial uniformity and extent must also be controlled in order to preserve semiconductor device yields.
A key process of semiconductor manufacturing is the creation of P-N junctions within the semiconductor substrate. This requires the formation of adjacent regions of P-type and N-type doping. An important example of the formation of such a junction is the implantation of P or N-type dopants into a semiconductor region already containing a uniform distribution of one dopant type. In these cases, an important parameter is the junction depth. The junction depth is defined as: the depth from the semiconductor surface at which the P-type and N-type dopants have equal concentrations. This junction depth is a function of the implanted dopant mass, energy and dose.
An important aspect of modern semiconductor technology is the continuous evolution to smaller and faster devices. This process is called scaling. Scaling is driven by continuous advances in lithographic process methods, allowing the definition of smaller and smaller features in the semiconductor substrate which contains the integrated circuits. A generally accepted scaling theory has been developed to guide chip manufacturers in simultaneously resizing all design aspects of the semiconductor device: i.e., at each technology or scaling node. The greatest scaling impact on ion implantation processes is the scaling of junction depths. This requires decreasing the junction depth as the device dimensions are decreased, requiring shallower junctions as integrated circuit technology scales. This translates into the following requirement: ion implantation energies must be reduced with each scaling step. The extremely shallow junctions called for by modern, sub-100 nanometer (nm) devices are termed “Ultra-Shallow Junctions”, or USJ.
Physical Limitations on Low-Energy Beam Transport
Due to the aggressive scaling of junction depths in CMOS processing, the ion energy required for many critical implants has decreased to the point that conventional ion implantation systems cannot maintain the desired wafer throughput. The limitations of conventional ion implantation systems at low beam energy are most evident in the extraction of ions from the ion source, and their subsequent transport through the implanter's beam line. Ion extraction is governed by the Child-Longmuir relation, which states that the extracted beam current density is proportional to the extraction voltage (i.e., beam energy at extraction) raised to the 3/2 power. Similar constraints affect the transport of the low-energy beam after extraction. A lower energy ion beam travels with a smaller velocity, hence for a given value of beam current the ions are closer together, i.e., the ion density increases. This can be seen from the relation J=ηeν, where J is the ion beam current density in mA/cm2, η is the ion density in ions/cm−3, e is the electronic charge (=6.02×10−19 Coulombs), and ν is the average ion velocity in cm/s. In addition, since the electrostatic forces between ions are inversely proportional to the square of the distance between them, electrostatic repulsion is much stronger at low energy, resulting in increased dispersion of the ion beam. This phenomenon is called “beam blow-up” and is the principal cause of beam loss in low-energy transport. Low-energy electrons present in the implanter beam line tend to be trapped by the positively-charged ion beam, compensating for space-charge blow-up during transport. Blow-up nevertheless still occurs, and is most pronounced in the presence of electrostatic focusing lenses, which tend to strip the loosely-bound, highly mobile compensating electrons from the beam. In particular, severe extraction and transport difficulties exist for light ions, such as the N-type dopants phosphorus and arsenic. Being lighter than arsenic, the phosphorus atoms penetrate further into the substrate than many other atoms, including arsenic. Hence the required implantation energies for phosphorus are lower than for arsenic. In fact, extremely low implantation energies, as low as 1 keV, are being required for certain leading edge USJ processes.
Heavier species, specifically cluster molecules, not only provide increased beam currents, but in many cases tend to amorphize the crystalline silicon lattice. This type of amorphization has been shown to be beneficial to the activation of P-type dopants such as boron, and should provide similar benefits for N-type dopants. Also, amorphization reduces ion channeling, enabling a shallower junction than possible in crystalline silicon. In fact, the process of record for many USJ logic manufacturers consists of a pre-amorphization implant of Ge or Si prior to performing the conductive doping implants in order to obviate channeling effects. The use of Ge or Si pre-amorphization implants has been shown to create end-of-range defects which result in increased leakage currents in the fabricated devices.
Molecular Ion Implantation
A technique to overcome the limitations imposed by the Child-Langmuir relation discussed above is to increase the transport energy is by ionizing a molecule containing the dopant of interest, rather than a single dopant atom. Upon entering the substrate, the molecule breaks up into its constituent atoms, sharing the energy of the molecule among the individual atoms according to their distribution in mass. While the kinetic energy of the molecule is higher during transport, the dopant atom's implantation energy is much lower than the original transport kinetic energy of the molecular ion. Consider the dopant atom “X” bound to a radical “Y” (disregarding for purposes of discussion the issue of whether “Y” affects the device-forming process). If the ion XY+ were implanted in lieu of X+, then XY+ must be extracted and transported at a higher energy. The increase is by a factor equal to the mass of XY divided by the mass of X. This ensures that the velocity of X in either case is the same. Since the space-charge effects described by the Child-Langmuir relation discussed above are super-linear with respect to ion energy, the maximum transportable ion current is increased. Historically, the use of polyatomic molecules to ameliorate the problems of low energy implantation is well known in the art. A common example has been the use of the BF2+ molecular ion for the implantation of low-energy boron, in lieu of B+. This process dissociates BF3 feed gas to the BF2+ ion for implantation. In this way, the ion mass is increased to 49 AMU from 11 AMU. This increases the extraction and transport energy by more than a factor of 4 (i.e., 49/11) over using single boron atoms. Upon implantation, however, the boron energy is reduced by the same factor of (49/11). It is worthy of note that this approach does not reduce the current density in the beam, since there is only one boron atom per unit charge in the beam. A detriment to this process is the implanting of fluorine atoms into the semiconductor substrate along with the boron. This is an undesirable feature of this technique since fluorine has been known to exhibit adverse effects on the semiconductor device.
Cluster Implantation
A more effective way to increase the dose rate is to implant clusters of dopant atoms. That is, molecular ions of the form XnYm+, where n and m are integers and n is greater than one. Recently, there has been seminal work using octadecaborane as a feed material for ion implantation. The implanted particle was a positive ion of the octadecaborane molecule, B18H22, which contains 18 boron atoms, and is therefore a “cluster” of boron atoms. This technique not only increases the mass of the ion and hence the transport ion energy, but for a given ion current, it substantially increases the implanted dose rate, since the octadecaborane ion B18Hx+ has eighteen boron atoms. By significantly reducing the electrical current carried in the ion beam (by a factor of 18 in the case of octadecaborane ions verses single boron atoms) not only are beam space-charge effects reduced, increasing beam transmission, but wafer charging effects are reduced as well. Since positive ion bombardment is known to reduce device yields by charging the wafer, particularly damaging sensitive gate isolation, such a reduction in electrical current through the use of cluster ion beams is very attractive for USJ device manufacturing. USJ manufacturing must accommodate increasingly thinner gate oxides and exceedingly low gate threshold voltages. Thus, there is a critical need to solve two distinct problems facing the semiconductor manufacturing industry today: wafer charging, and low productivity in low-energy ion implantation. A favorable attribute is the self-amorphizing feature in the implant as described earlier. As discussed below, the present invention increases the benefits of N-type cluster implantation by using significantly larger phosphorus or arsenic clusters having more than 3 dopant atoms.